This application relates to the conversion of light irradiation to electrical energy, and more particularly, to methods and tools for producing bifacial photovoltaic devices (e.g., bifacial solar cells) and arrangements of bifacial devices (e.g., bifacial solar cell modules) that convert solar energy to electrical energy.
Solar cells are typically photovoltaic devices that convert sunlight directly into electricity. Solar cells commonly include a semiconductor (e.g., silicon) that absorbs light irradiation (e.g., sunlight) in a way that creates free electrons, which in turn are caused to flow in the presence of a built-in field to create direct current (DC) power. The DC power generated by several PV cells may be collected on a grid placed on the cell. Current from multiple PV cells is then combined by series and parallel combinations into higher currents and voltages. The DC power thus collected may then be sent over wires, often many dozens or even hundreds of wires.
Presently, the majority of solar cells are manufactured using a screen printed process which screen prints front and back contacts. The back contact is commonly provided as a layer of aluminum. The aluminum layer will cover most if not all the back layer of the silicon wafer, thereby blocking any light which would reflect onto the back surface of the silicon wafer. These types of solar cells therefore receive and convert sunlight only from the front exposed surface.
However, another type of known solar cell is a bifacial solar cell, which acquires light from both surfaces of the solar cell and converts the light into electrical energy. Solar cells which are capable of receiving light on both surfaces are available on the market. One example is the HIT solar cell from Sanyo Corporation of Japan, as well as bifacial solar cells sold by Hitachi Corporation, also of Japan.
Drawbacks with existing bifacial solar cells include those related to the manufacturing processes. Various ones of these drawbacks are similar to those drawbacks existing in the manufacture of single-sided solar cells, such as discussed, for example, in U.S. patent application Ser. No. 11/336,714, previously incorporated herein by reference. As discussed in that document, desired but largely unavailable features in a wafer-processing tool for making solar cells are as follows: (a) never breaks a wafer—e.g. non contact; (b) one second processing time (i.e., 3600 wafers/hour); (c) large process window; and (d) 24/7 operation other than scheduled maintenance less than one time per week. The desired but largely unavailable features in a low-cost metal semiconductor contact for solar cells are as follows: (a) Minimal contact area—to avoid surface recombination; (b) Shallow contact depth—to avoid shunting or otherwise damaging the cell's pn junction; (c) Low contact resistance to lightly doped silicon; and (d) High aspect metal features (to avoid grid shading while providing low resistance to current flow).
It is particularly desirable to provide feature placement with high accuracy for feature sizes below 100 microns. By minimizing the feature sizes, more surface area is available for the accumulation and conversion of solar light. Features on the order of 10 microns or smaller can suffice for extracting current. For a given density of features, such a size reduction may reduce the total metal-semiconductor interface area and its associated carrier recombination by a factor of 100.
Further, a major cost in solar cell production is that of the silicon layer itself. Therefore, the use of thinner layers is desirable as one way of reducing costs associated with the manufacture of solar cells. However, with existing technology, the manufacture of thin crystalline (silicon) layers (e.g., 150 microns or less) is not commercially feasible, if not impossible, due to the previously mentioned unavailable features, and because the contact layers such as silver, aluminum, etc., cause the semiconductor layers to warp or bow.
In addition, such thin devices in general have a problem that not all light is absorbed by the thin cell. To reach high efficiency using a thin silicon layer, cells require a design which permits a higher percentage of the light to be absorbed. Ideally, a high efficiency thin cell of any material in construction will accept light incident on it from either side with minimal loss, and then trap the useful portion of the solar spectrum so that it is absorbed to create photovoltaic energy.